Morphology

SECTION I MOEPHOLOGY The object of vegetable morphology is the scientific study of the forms of plants. It does not attempt to discover the causes of the , development of specific forms, but rather has accomplished its purpose when it succeeds in showing how one form may be derived from another. The only real basis of morphological study is, accordingly, the genealogical development or phylogeny (p. 1). As phylogenetic development can only be inferred, and cannot be directly followed, the methods of morphology must also be indirect. They are de- pendent on the one hand upon ontogeny, i.e. on the study of the development passed through by an organism in attaining its mature condition, and on the other hand upon the comparison of existing organisms with one another and with those that have become extinct. To a certain extent the ontogenetic development of a plant repeats its i^hylogeny and helps to elucidate the latter, while, by means of comparative investigation, extreme forms may be connected by inter- mediate links. As, however, the ontogeny of a plant is neither a complete nor invariable repetition of its phylogeny, and as connecting links between extreme forms are often Avanting, the results of morpho- logical study are frequently incomplete. Such parts or members of plants which it is reasonable to presume have had a common origin are distinguished as homologous ; those which, while probably having different origins, yet exercise the same functions, are termed ANALOGOUS. Through the adaptation of different parts to the same function, a similarity in both external form and internal structure often results ; and in this way the correct determination of morpho- logical relationships is rendered difficult. Only homologous parts have the same " morphological value." lliis homology is determined by the facts of phylogeny and origin, and not by any correspondence in function. Though the function of any structure does not influence 9 10 BOTANY PART I its morphological value, the need of making clear the intimate connection between form and function often introduces jDhysiological considerations into morphological questions. When, for phylogenetic reasons, it seems possible to attribute to a number of different members a common origin, such a hypothetical original form is termed the fundamental or primitive form. The various modi- fications which the primitive form has passed through constitute its METAMORPHOSIS. In this way the theory of the metamorphosis of plants acquires an actual significance. Slightly differentiated structures, which are found at the beginning of a series of progressively differentiating forms, are termed RUDI- MENTARY ; imperfect structures, which have arisen as the result of the deterioration of more perfect forms, are termed REDUCED. Vegetable morphology includes the study of the external form and the internal structure of plants. The descriptive study of the external form of plants has been termed organography. This term will not be used, since by the use of the word " organ," it Avould seem to have a physiological signification. Morphology takes no recognition of the parts of a plant as organs, but treats of them merely as members of the plant body. On the other hand, one of the most important aims of physiology is to place the external form and the internal structure of the living body in relation to the functions performed by the latter; physiology also investigates the causes of the organisation. The study of the internal structure of plants is often designated Anatomy or Phytotomy ; but as it mainly deals with the study of the more minute internal structure, it resembles rather histology, in the sense in which that term is used by zoologists, and concerns itself to a much less degree with the coarse anatomy of the plant body. In any case, it is the simplest plan to designate the study of the outer form external morphology, and that of the inner structure internal morphology. O I. EXTERNAL MORPHOLOGY Plants show a great diversity in the form and arrangement of their members ; it is the task of morjihology to determine the points of agreement existing between them. It seeks to do this by determin- ing the common origin of the homologous parts or members. The Development of Form in the Plant Kingdom -- The Thallus {^"). When the body of a plant is not differentiated into ^epaiatc mcmbei's, or is composed of members which (though they may be similar) are not homologous with those of the most highly SECT. I MORPHOLOGY 11 o^ganised plants, it is termed a thallus. When the thaUus is differentiated into members analogous to those of the higher plants some confusion may arise from the same names being used for parts x^ which, since their origin has been distinct,' - not , homolo^gjojwuisiu.. are ^'�cerev2i-s-isa�e�.'^�''1,<'TMC2e/l^lfs� without buds; 2 and The simplest form 3, budding cells, (x that we can imagine for ^'^'^ an organism is that of a sphere, and this -- Fk;. 1. Gloeocapsa polydermatim. A, Connnencenient of divisioii ; B, (to the left) shortly after division; C, a resting stage, (x 540.) is actually the form of some of the lower A plants. grCCU grOWth oftCn SCCn On ^ the ^ ^^^jj^ . consists of . an aPggre. ga,ti, on, of microscopically small spherical bodies of Gloeocapsa polydermatica (Fig. 1), an Alga belonging to one of the lowest divisions of the vegetable kingdom. The single plants of the Beer-yeast (Saccharomyces cerevisiae) are ellipsoidal ; but, from their o /V-3 -- Fio. 4. Bacteria from deposits on teeth, a, Leptothrix hiiccalis; a, the same after treat- ment with iodine ; 6, Micrococcus ; r, Spiro- cliaete dentium after treatment with iodine ; d, Spirillum sprUigenum. (x 800.) -- Fig. 3. Pinnularia viridis. A, Surface view ; B, lateral view, (x 5-tO.) peculiar manner of growth, by budding, they form lateral outgrowths, and thus often appear constricted (Fig. 2). Cylindrical and also discshaped forms are shown by various Algae. The Diatomeae (Fig. 3), 12 BOTANY PAET I ill particular, exhibit a great variety of spindle, canoe, helmet, and fan-like shapes ; but they may all be derived from the more simple spherical, discoidal, or cylindrical forms. Among the ^^ Bacteria, Avhich, as the cause of infectious diseases and of decomposition, have been the object of so much recent investigation, we also meet with spherical, rod-shaped, filamentous, and spirally wound forms (Fig. 4). The next stage in the progressive development of external form in the vegetable kingdom is exhibited by such plants as -- Fi(!. 5. UIca Lftctuca, young stage, show- ing apex and base. (X 2-20.) C-- Fio. Portion of Cladophora glomerata. (X 48.) Fig. 7.--Cladostephus verticillatvs. (After Prinosiieim, X 30.) show a DiFFEHKNTfATioN INTO APEX AND BASE. The base serves as a point of attachmojit, while growth is localised at the apex. In this way a growing i)oint is developed at the apex. As an example of such a form, a young plant of the green Alga, Ulva Laduca (Fig. 5), SECT. I MORPHOLOGY 13 may be taken. The development of a more complicated external form is represented by the branched filamentous, or ribbon-shaped Algae, in which the origin of new formations is more and more restricted to the apex. An ACROPETAL order of development, in which the youngest lateral members are always nearest the growing apex, is clearly shown by the branched filaments of the common green Alga, Cladophora glomerata (Fig. 6). Still moi^e pronounced is the apical growth in the brown seaweed Cladosfephus -- Fii;. S. Didi/ota diehotoma. (g iiat. size.) verticUlatus {Fig. 7). The great variety in the form of the larger Fungi and Lichens, by which they are distinguished as club-, umbrella-, salver-, or bowl-shaped, or as bearded or shrub-like, comes about by the union or inter- twining of apically growing filaments. This type of con- struction is limited to Fungi and Lichens. As the apex itself may undergo successive bifurcation, as in the case of Didyota dichotoina (Fig. 8), it does not always necessarily follow that new members must be formed beneath the original apex. The highest degree of ex- ternal differentiation among the lower plants is met with in certain groups of Eed and Brown Sea-weeds (Rhodophyceae and Phaeophyceae). Some representatives of these classes resemble the higher plants in the formation and -- Fin. 9. Hydrolapathum sanguineum. {K nat. size.) arrangement of their members ; Hydrolapathum sanguineum (Fig. 9), for example, as is indicated by its name, resembles a species of liumex, and affords an instructive illustration of the analogy of form existing between plants phylogenetically widely distinct from one another. 14 BOTANY PART I -- The Cormus. All plants the body of which can be regarded as a tliallus are grouped together as Thallophytes, and contrasted with Cormophytes in Avhich a shoot is developed. The latter include two developmental series which, though assumed to have had a common cCV^ -^ 1 -- Fig. 10. Riccin Jliiitans. (Nat. size.) Fio. Il.--Blasia pnsilla. s, Sporogonium ; r, rhizoids. (x 2.) Starting-point, have attained a cormophytic organisation independently. These two groups are the moss-like plants or Bryophyta and the Pteridophyta including the ferns and their allies ; from the Pterido- phyta the seed-plants, the highest group in the vegetable kingdom was derived. Among the Bryophytes forms occur, as in some groups of Liverworts (Hepaticae), with a completely thalloid body which is ribbon-shaped and only exhibits an imper- fect differentiation into members. Thus Fl�i. \2.--Pla{iiochUii. asplenioides. g, siiorogonium. (Nat. size.) Eiccia fluitans (Pig- 10) is ribbon-shaped and dichotomously branched, and its habit or general appearance recalls the Brown Alga Didyota dichotoma mentioned above (Fig. 8) as an example of a thallus. Blasia pusilla (Fig. 11) has marginal indentations in its ribbon-shaped body. Lastly, Flagiochila asplenioides (Fig. 12), another Liverwort, has a distinction of stem-like and leaf -like members which is completely analogous to that exhibited by the most highly organised plants. In addition to the distinction of stem and leaf in their .shoots, the Pteridophyta possess true roots, while even the most highly oi'ganised Bryoi)hyta have only filamentous structures (rhizoids) (Fig. 11 r) in place of roots to attach them to the substratum. True roots, on the other hand, which appear for SECT. I MORPHOLOGY 15 the first time in the Pteridophyta, are, for the most part, cylindrical structures with apical growth. Besides possessing a distinctive internal construction they are distinguished in their external form from the shoot by having a special sheath, the ROOT-CAP or CALYPTRA -- covering the growing point, and by the absence of leaves. The Metamorphosis of the Primary Members of Cormophytes. After the differentiation into shoot and root had taken place, further changes have consisted essentially in a more or less profound modification of these primary members of the cormophytic plant-body. Such -- changes are spoken of as a metamorphosis (p. 10). Members of Independent Origin. Parts which cannot be derived by metamorphosis of the primary members of the cormophytic plant are sometimes met with. Though they are of infrequent occurrence they are of importance as showing that the natural evolutionary process is not to be limited by any formal scheme. Such structures will be discussed farther on. The relationships between homologous members, which are often very striking, did not escape the notice of earlier observers. They suggested comparisons, although no real phylogenetic liasis for such comparisons existed. Thus, an idealistic conception of the form of external members was develo23ed, and reached its scientific conclusion in the writings of Alexander Beaun. As the great variety exhibited in the external appearance of the lower plants precluded any possibility of assigning to them hypo- thetical primitive forms, the whole terminology of the external morphology of plants has been derived from concej^tions applicable only to the Cormophytes. Even to-day, the same terms used in reference to the Cormophytes are applied to parts of the Thallophytes, which are evidently only analogous. On this account in treating here of the nature and aim of general morphology examples will be taken mainly fi'bm the Cormophytes. Relations of Symmetry Every section through a part of a plant, made in the direction from base to apex, is distinguished as a longitudinal section ; those at right angles to it being termed cross or transverse sections. Parts of plants which may be divided by a number of longitudinal planes into like halves are termed either multilateral, radial, or actino- MORPHIC. Such parts are symmetrically constructed around their longitudinal axis. The degree of symmetry peculiar to any leafy shoot will be more apparent from a diagram, that is if the leaves which it bears be projected on a plane at right angles to its axis. The radial symmetry of a shoot with opposite leaves in alternating A whorls is clearly shown in the adjoining diagram (Fig. 13a). 16 BOTANY PART I shoot witli its leaves arranged alternately in two rows shows somewhat different relations of symmetry. The diagram of such a shoot -- Fk;. 13a. Diagram showing the so-called de- cussate arrangement of leaves. -- Fi(i. 13b. Diagram showing two-ranked alternate arrangement of leaves. (Fig. 13b) can only be divided into similar halves by two planes. When such a condition exists, a member or plant is said to be BILATERAL. When, however, a division into two similar halves is only possible in one plane, the degree of symmetry is in- dicated by the terms dorsiventral or ZYGOMORPHic ; since, while the right and left halves correspond to one another, difterences exist between the dorsal and ventral surfaces. Ordinary foliage -leaves exhibit this dorsiventral structure. In the accompanying figure (Fig. 14) such a monosymmetrical, dorsiventral foliage -leaf is diagrammatically represented. From the surface view (A) and from the crosssection {B), in which the distinction betw^een the dorsal and ventral sides is indicated by shading, it is obvious that B ^v\y!nimi\m\ nm u, but one plane of symmetry (.s) can be drawn. Dorsiventral members are often asymmetrical, not being divided by any i plane into corresponding halves : the leaves -- of many kinds of Begonia will serve as Fi.^ 14.-- Diagram of a foliage-leaf, examples of this. In such cascs and -- i.trr::,-;:::'fs^mm:Sr^^^ the leaf of the Elm may be mentioned as another striking example the asym- metry of the indi\idiial leaf is subordinated to the symmetry of the entire plant. SECT. I MORPHOLOGY 17 Branch Systems Thallophytes as well as Cormopliytes exhibit systems of branching, resulting either from the formation of new growing points by the bifurcation of a previously existing growing point, or from the develop- ment of new growing points in addition to those already present. In this way there arise two systems of branching, the dichotomous and the MONOPODIAL. By the uniform development of a continuously bifurcating stem, a typical dichotomous system of branching is Fifi. 15.-- Diagrams of branch systems. A, Dicliotomous branching; Au, eqnal dichotomy; Ah, scorpioiil dichotomy ; Ac, helicoid dichotomy. B, monopodial branching ; Ba, false dichotomy ; Bh, scorpioid cyme ; Be, helicoid cyme ; s, s, sympodia. produced, such as is shown in Didyota dichotoma (Fig. 8), and is represented diagrammatically in Fig. 15 Aa. In a typically developed example of the monopodial system there may always be distinguished a persisting main axis, the MONOPODIUM, giving rise to lateral branches from which, in turn, other lateral branches are developed. Good examples of this form of branching are afforded by many Conifers such as the Onjptomeria represented in Fig. 16. Where one of the two branches is regularly developed at the expense of the other, the dichotomous system assumes an appearance quite different from its typical form. The more vigorous branches may then, apparently, form a main axis, from which the Aveaker branches seem to spring, just as if they were lateral branches. This mode of branching (Fig. t c 18 BOTANY PART I 15 Ah) is illustrated by the Selaginellas. axis (.<;, s) is termed, in accordance Avith its Such origin, an apparent a SYMPODiuar. main On the other hand, in the monopodial system two or even several lateral branches may develop more strongly than the main axis, and so simulate true DICHOTOMY or POLY- TOMY. Such mono- podial forms of branch- ing are referred to as FALSE DICHOTOMY (Fig. 15 Bi) or false POLYTOMY, as the case may be. A good ex- ample of false dicho- tomy may be seen in the Mistletoe (Fiscum album). If, however, a lateral branch so ex- ceeds the main axis in development, pushing the apex of the latter to one side, that it seems ultimately to become a prolongation of the axis itself, a sympodium is again formed (Fig. 15 Bh). This is what occurs in many of our forest trees, e.g. the Lime and Beech in both of ; these trees the ter- minal buds of each year's growth die, and -- Fio. 10. Cryptomeria japonica, Don. (From L. Beissner, Hand- the prolongation of huch tier Nailelholzkunde. Greatly reduced.) the stem, in the follow- ing spring, is continued by a strong lateral bud, so that in a short time its sympodial origin is no longer recognisable. In many rhizomes, on the other hand, the sympodial nature of the axis can be easily distinguished ; as, for example, in the rhizome of Polt/r/onatum muliifloruiii (Fig. 23), in which, every year, the terminal bud gives rise to an aerial shoot. SECT. I MORPHOLOGY 19 Avhile an axillary bud provides for the continuance of the axis of the rhizome. In the flower-producing shoots or inflorescences of Phanerogams the different systems of branching assume very numerous forms. These will be more fully described in their proper place. The Shoot -- The Development of the Shoot. ^ Under the term shoot a stem A and its leaves are collectively included. stem possesses an apical mode of growth (Fig. 17), and its unprotected growing point is described as naked, in contrast to that of the root with its sheathing root-cap. The apex of the shoot generally terminates in a conical protuberance, called the vegetative CONE. As it is usually too small to be clearly visible to the unaided eye, it is best seen in magnified median longitudinal sections. So long as the apex of the shoot is still internally undifferentiated, it continues in the embryonic condition, and it is from the still embryonal vegeta- tive cone that the leaves take their origin. They first appear in acropetal succession as small, coni- cal protuberances, and attain a larger size the farther removed they are from the apex of the stem. As the leaves usually grow more rapidly than the stem which produces them, they envelop the -- Fig. 17. Apex of a shoot ofa phanerogamic plant. V, Vegetative cone ; /, leaf rudiment ; g, rudiment of an axillary bud. (x40.) more rudimentary leaves, and, overarching the vegetative cone, form a BUD. Buds are therefore merely undeveloped shoots. If they are to remain for a long time undeveloped, as for example is the case with winter buds, they are -- protected in a special manner during their period of rest. The Origin of New Shoots. The formation of new growing points by the bifurcation of an older growing point, in a manner similar to that already described for Didyota dichotoma (Fig. 8), occurs also in the lower thalloid Hepaticae (lUccia fluituns, Fig. 10). Among the cormophytes this method of producing new shoots is of less fre- quent occurrence, and is then mainly limited to the Pteridophytes, and is typically shown only in some Lycopodiaceae. In this case, whenever a shoot is in process of bifiu'cation, two new vegetative cones are formed by the division of the growing point (Fig. 18). In most of the Lycopodiaceae the new shoots thus formed develop 20 BOTANY PART 1 unequally ; the weaker becomes pushed to one side and ultimately appears as a lateral branch (Fig. 19). Although a relationship as regards position is generally apparent between the origin of leaves and the lateral shoots, in the system of branching resulting from such a liifurcation of the vegetative cone this connection does not exist. In the more highly developed Bryophytes, particularly in the true ]\Iosses, new shoots arise obliquely below the still rudimentary leaves at some distance from the growing point. In the Phanerogams new shoots generally arise in the axils of the leaves. In the accom- panying illustration of a longitudinal section of a phanerogamic shoot (Fig. 17) the rudiment of a shoot (g) is just appearing in the axil of the third uppermost leaf ; in the axils of the next older leaves the conical protuberances of the embryonic leaves are already beginning to appear on the still rudimentary shoot of the bud. Shoots thus produced in the axils of leaves are termed AXILLARY SHOOTS. The leaf, in the axil of Avhich a shoot develops, is called -- Fio. 18. Longiturlinal section of a bifurcat- ing shoot (j)) of Lycopodium alpinum, showing equal development of the rudi- mentary shoots, p', p" ; h, leaf rudiments ; c, cortex ; /, vascular strands. (After Heoelmaier, X 60.) -- Fig. 19. Bifurcating shoot (j') of LycojwdUnn inundatum, showing unequal development of the rudimentary shoots, p', p" ; h, leaf rudiments. (After Heoelmaier, x 40.) its SUBTENDING LEAF. An axillary shoot is usually situated in a line A-ith the middle of its subtending leaf, although it sometimes becomes pushed to one side. As a rule, only one shoot develops in the axil of a leaf, yet there are instances where it is followed by additional or ACCESSORY shoots, which either stand over one another (serial buds), as in Lorncera, Gkditschia, Gymnodadus, or side by side (collateral buds), as in many Liliaceae, e.g. species of Allium and Muscari. Although in the vegetative regions, i.e. the regions in which merely vegetative organs are produced, the rudiments of the new shoots of phanerogamic plants make their appearance much later than those of the leaves, in the generative or flower-producing regions the formation of the shoots follows directly upon that of their sub- tending leaves, or the shoots may even precede the leaves. In this last case the subtending leaves are usually either poorly developed or completely suppressed, as in the inflorescence of the Cruciferae. SECT. I MORPHOLOGY 21 The bud forming the growing end of a shoot is called the terminal bud, while those borne on the sides of the shoot are the lateral buds. Shoots developing in predetermined positions on young parts of the plant are designated normal, in contrast to adventitious SHOOTS, which are produced irregularly from the old or young portions of a plant. Such adventitious shoots frequently spring from old stems, also from the roots of herbaceous plants (Brassica oleracea, Anemone sylvestris, Convolvulus arvensis, Eumex Acetosella), or of bushes {Euhus, Rosa, Corylus), or of trees (Populus, Ulmus, Bobinia). They may even develop from leaves, as in Cardamine pratensis, Nastwrimm officinale, and a number of Ferns. An injury to a plant will frequently induce the formation of adventitious shoots, and they frequently arise from the cut surface of stumps of trees. Gardeners often make use of pieces of stems, rhizomes, or even leaves as cuttings from which to produce new plants. Leaves and also normal shoots, which make their appearance as outgtorowths from the portions of the parent shoot, still in an embryonic condition, have an external or exogenous origin. Adventitious shoots, on the other hand, Avhich arise from the older parts of stems or roots, are almost always endogenous. They must penetrate the outer portions of their parent shoot before becoming visible. Adventitious shoots formed on leaves, however, arise, like normal shoots, exogenously. Buds are formed in the marginal indentations of tlie fleshy leaves of species of Bryoiihyllum (Crassulaceae). Although arising from the leaf these buds must properly be regarded as "normal," and as forming part of the normal ontogeny of the plant, since they arise in }>re-determined positions from young tissue. In the strict sense of the term only those buds can be called adventitious which are produced in casual positions from tissues which in their production enter into renewed activity, e.g. the